1. Field of the Invention
The present invention relates to a voltage control circuit for a common mode voltage and a method for controlling the same. More specifically, the present invention relates to a voltage control circuit of a common mode voltage for an operational amplifier or an operational transconductance amplifier (hereinafter abbreviated as OTA) which has differential outputs and controls the common mode voltage of the differential outputs, and to a method for controlling the common mode voltage.
2. Description of the Related Art
As an integration of semiconductor integrated circuits becomes higher and digital technologies develop more advancement in recent years, circuits with digital and analog circuits formed in a large scaled integrated (LSI) chip have been mounted in many consumer products.
Moreover, in the field of analog circuits, circuits such as operational amplifiers or OTAs having differential outputs are well known. For the circuit shaving the differential outputs, many circuits with various configurations have heretofore been disclosed as a circuit for controlling a common mode voltage of the differential output.
FIG. 1 is a circuit block diagram showing an example of a configuration of a conventional common mode voltage setting circuit described in Document 1. This conventional common mode voltage setting circuit detects a common mode voltage of differential output terminals using a detection circuit 203.
The detected common mode voltage VCM from the detection circuit 203 is inputted to one of input terminals of an operational amplifier 204 and a constant reference voltage VCM1 is also inputted to the other input terminal of the operational amplifier 204. The output terminal of the operational amplifier 204 is connected to the control terminals flowing through an OTA1 (201) and an OTA2 (202) both with a differential output. The common mode voltage is controlled by the operational amplifier 204 and is set at a desired voltage by setting driving currents of OTA1 (201) and OTA2 (202) through the current mirror circuits.
A control signal loop for setting the common mode voltage is included in the foregoing circuit configuration, in addition to an ordinary signal loop for processing an input signal to output it as a desired signal. The OTAs whose transconductances are proportional to their driving currents or a square-root of their driving currents substantially function as multipliers which receive two kinds of signals, the input signal and a control signal. Therefore, with regard to the control signal loop, influence of the ordinary signal loop appears therein, which makes it difficult to keep a phase margin or a gain margin for the control signal loop.
Moreover, in order to reduce a set voltage error of the common mode voltage setting circuit, a gain of the control signal loop should be set higher. However, when the gain is set higher, circuit operation becomes unstable because the phase margin for the control signal loop becomes smaller, thereby causing a problem that an oscillation phenomenon appears in the control signal loop. On the other hand, when the gain of the control signal loop is set lower so that the circuit operates stably, the setting error of the common mode voltage on an output side cannot be made to be a desired value or less.
[Document 1]
J. Silva-Martinez, M. S. J. Steyaert, and W. Sansen, Design Techniques for High-Performance Full-CMOS OTA-RC Continuous-Time Filters, IEEE Journal of Solid-State Circuits, Vol. 27, No. 7, pp. 993–1001, July 1992.
The present inventor has recognized that, in the conventional circuit configuration, the operational amplifier or the OTA, which are to be controlled, functions as a multiplier that receives two kinds of signals, the input signal and the control signal, in order to secure the phase margin or the gain margin as large as possible. Therefore, an influence of the input signal appears in the control signal loop, it makes the control signal loop very difficult to secure the phase margin or the gain margin.
Moreover, when the gain of the control signal loop is reduced in order to secure the phase margin of the control signal loop, the setting error becomes large. To alleviate this problem in the conventional circuit configuration as much as possible, the driving current of the operational amplifier should be increased to raise the operating frequency characteristic of the control signal loop thereof. Therefore, it is difficult to reduce current consumption of the circuit.
The present invention is made in consideration of the above conventional technology. An object of the invention is to provide a voltage control circuit for a common mode voltage capable of effectively controlling the common mode voltage of the differential output.